The hydrocarbon steam reforming process, commonly known as SMR (“steam methane reforming”), employs a reforming reaction which produces hydrogen and carbon monoxide from hydrocarbons and steam. This reaction entails a significant influx of heat because of its highly endothermic nature. To this end, the hydrocarbons and the steam are introduced into catalyst-filled reforming tubes, said tubes being placed in a furnace. The mixture obtained at the outlet of the tubes is predominantly made up of H2 and CO, known as synthesis gas or syngas.
The furnace is made up of one or more combustion chambers made of refractory walls and of burners placed on these walls. The burners are arranged in such a way as to transfer the heat of their combustion to the mixture of hydrocarbons and of steam through the wall of the tubes, generally by radiating the heat of the flame onto the refractory walls of the combustion chamber. The present invention relates to furnaces in which the burners are positioned on two of the vertical walls of the combustion chamber, said walls, known also as side walls, being opposing walls facing one another. This type of furnace is heated from the side (side-fired) and differs from furnaces in which the burners are mounted in the roof of the furnace (known as top-fired furnaces).
In side-fired furnaces, in each combustion chamber, the reforming tubes are arranged in a line in a plane situated equidistant between the two vertical side walls that support the burners. The combustion gases, also known as flue gases, are discharged from the seat of combustion in the top part of the furnace (the top part or top region of the furnace means that region of the furnace that is situated above the highest row of burners). The outlets, at the furnace end, of the flue gas discharge pipes are located at the top end of the furnace on just one of the side walls. There may be a number of flue gas discharge pipes in one and the same furnace but these pipes are always positioned on one and the same vertical side wall, at the same height in the furnace, and are uniformly spaced apart.
It has been noticed that arranging the flue gas discharge pipes asymmetrically in the furnace in relation to the row of tubes causes preferential aspiration of the combustion gases in that half of the furnace that is situated on the same side as the outlets of said pipes (we shall also call this the 1st half of the furnace); this preferential aspiration leads to asymmetry in the flow of combustion gases and leads to a loss of thermal efficiency of the furnace.
What specifically happens is that it leads to significant asymmetry in the range of temperatures of the combustion gases on each side of the row of tubes which results from the hot gases passing preferentially toward the outlet in said 1st half of the furnace (situated on the outlet side). In the 2nd half of the furnace, the combustion gases suffer a pressure drop which can be attributed to their need to pass across the row of reforming tubes in order to reach the outlet. Because the obstacle that the row of tubes represents considerably slows the passage of the combustion gases, these gases begin to recirculate in the top end of this 2nd half of the furnace. The residence time of the combustion gases as they cross the top region of the furnace is therefore shorter in the 1st half than in the 2nd half.
However, the combustion gases with the shorter residence time transfer less energy by radiation to the row of forming tubes and exit into the outlet pipes at a hotter temperature. The thermal energy discharged as a result of a higher flue gas temperature thus represents a synthesis gas production potential that is not realized.